This invention relates to a semiconductor device, and more particularly, to such a device which includes a specially doped region which is provided to prevent electrical breakdown under certain operating conditions. For the purpose of illustration, a preferred embodiment of the invention is described in conjunction with a field-effect, power-MOS type transistor wherein the invention has been found to have particular utility.
In the packaging of a semiconductor device, such as a field-effect, power-MOS transistor, it is, of course, necessary to make electrical contact (through leads) to the several electrodes (gate, source, drain) in the device. Presently, the best-available wire-bonding technology to achieve this requires, because of the relatively gross size of even the smallest wire which is practically handleable, enlarged bonding pad areas in a device to provide an appropriate contact expanse for a wire lead. In power-MOS transistors, one of such enlarged bonding areas is provided for the gate, and inclusion of such an area, under certain circumstances, presents both electrical and mechanical problems.
The invention disclosed and claimed herein relates to a structural modification, through special doping, in a semiconductor device to deal with the electrical problem. Another invention of mine, which is disclosed in a companion U.S. patent application, Ser. No. 842,556, filed Mar. 21, 1986, (now abandoned) for "Semiconductor Device with Anti-Fracture Support Structure" addresses the mechanical problems.
Turning attention now to the electrical problems just referred to, for a given desired operational switching voltage in a device such as a field-effect, power-MOS transistor, there is a maximum allowable spacing between what might be thought of as the electrical breakdown-prone zones in order to assure that breakdown does not occur. In a device of the type just mentioned, the zones referred to take the form of the so-called channels in the device. In a typical power-MOS transistor, a region which is problematic is that region which extends as an expanse beneath the gate contact area, which region is at least partially bounded by channels whose separation greatly exceeds the maximum permissible spacing to prevent breakdown. For example, in a power-MOS transistor designed to operate with a switching voltage of 500-volts, the maximum allowable adjacent channel spacing is about 2-mils, yet in the gate contact area, it would be typical to find an adjacent channel spacing of about 10-30-mils. As was mentioned earlier, this greater spacing results as a consequence of having to provide a large enough contact area to enable a wire lead to fit.
Accordingly, a general object of the present invention is to provide an improved semiconductor structure which, while allowing wide spacing between breakdown-prone zones to accommodate lead attachment, nevertheless substantially completely solves and avoids the problem of electrical breakdown.
In accordance with a preferred embodiment of the invention, and as will be described more fully below, the invention proposes the inclusion of a specially doped region between such zones which, in effect, establishes an electrical connection with the zones in a manner which obviates electrical breakdown. In the case of a field-effect, power-MOS transistor, and referring for a moment specifically to what is known as an N-channel device where the channels contain a P- dopant, the invention includes a P- doped subregion extending between the adjacent channels underlying the gate contact area, which subregion makes an electrical connection, and preferably a conductive connection, between the channels. This solid-state structure eliminates the build-up and concentration of electrical fields which can cause debilitating electrical breakdown.
While there are certainly different ways in which such a specially doped subregion can be established, the description which follows below sets forth a preferred manner of creating such a subregion.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.